Q. We are continually running up against structural engineers who push the limits of the cover for rebar and post-tensioning cables for suspended slabs in concrete structures. We normally never like to work with rebar cover less than 1/4 inch above the aggregate size.
We are finishing an underground parking garage project where some of the floors lack sufficient concrete coverage on the bottom bars of a suspended, post-tensioned slab. The structural engineer specified 3/4-inch cover for the bottom bars and a 3/4-inch aggregate mix.
The reinforcing steel sagged during placement between the support chairs, causing the 3/4-inch spacing to be infringed upon. Rust is visible where some of the bottom bars are at the bottom of the slab, and some rebar ridges are faintly visible.
The building is new and we’d like some cost-effective options to solve this problem. Because the building is new, there shouldn't be any concern with deterioration of the rebar. This issue needs to be addressed to arrest the corrosion. The decks are not exposed to weather, as they are below grade with five floors of a building on top and will only see water that comes in on vehicle tires. We normally deal with this issue on the deck surface, not at the soffit, so placing a topping is not an option. Any ideas?
A. First, let’s address the issue of minimum cover, which is specified in Section 7.7 of ACI 318, Building Code Requirements for Structural Concrete. Significantly, that section is titled “Concrete Protection for Reinforcement” and it addresses avoiding water migration and subsequent damage to the rebar.
The minimum concrete cover requirements are based on exposure conditions and rebar size. In your case, the slab is not exposed to weather or in direct contact with ground, allowing for the 3/4-inch cover, provided the rebar is No. 11 or smaller.
However, Section 7.5, “Placing Reinforcement,” specifically permits only a minus 1/4-inch in concrete cover for soffits such as you describe. It sounds like that tolerance was not met.
Guidance on the size of aggregate relative to minimum cover is found in ACI 318, Section 3.3, “Aggregates.” Although not specifically addressed, the intent is clear: The aggregate shouldn’t be large enough to impede consolidation and lead to honeycombs or voids. In this project, the end product would likely have been much more satisfactory if additional cover had been specified.
You might consider a migrating corrosion inhibitor (MCI) treatment. This takes advantage of the ability of liquids to penetrate into concrete. These materials — available as caulks, liquids, or gels — are applied to the concrete surface or injected into holes drilled into the slab. From there, the corrosion-inhibiting chemicals migrate toward, and ultimately protect, the rebar, similar to the protection afforded by corrosion-inhibiting admixtures added to fresh concrete.
Choosing which application technique to use depends on site conditions and protection requirements, and the material to be used should be chosen accordingly.